Antenna system



D. E. SPARKS Nov. 3, 1942.

ANTENNA SYSTEM Filed Feb. 23, 1940 R N Z pd.- I 4 m J J w z a h w 1 V Hm0 1 Z fi 5. j /J M Fllll. E1 2 7 o W/IX/QQ MM W 3 I ll||llmnwmwwwwdww fij Q 2 0 5 7L2 z 1,. 2 s z 2 1 god Aw J V n01 a .a I, I] H WW Z a 7Patented Nov. 3, 1942 U E STTES PTENT OFFICE Radio Corporation, ofIllinois Chicago, 111., a corporation Application February 23, 1940,Serial No. 320,247

3 Claims.

This invention relates to radio antenna systems and in particular todouble loop antenna systems.

With progress in the radio art evidencing itself, in more sensitive andselective receivers, there has been likewise public demand forincreasingly improved quality in reproduction. In many instances, theimprovement in the radio receiver circuit has been lost, by the radiouser connecting the receiver to an inadequate or improperly designedantenna system, which in turn has seriously affected the reproductionfrom the receiver. In attempting to counteract this difliculty and atthe same time to fulfill the demand for greater compactness andportability in radio apparatus, loop antennae have been incorporateddirectly into the receiving housing, or have been connected with thehousing in such a way as to be moved therewith.

Single loop antenna systems have the characteristic feature of effectingan induced voltage, due to an oncoming wave, which is directlyproportional to the frequency of such wave. This characteristic gives arising voltage over the broadcast or receiving range so that the signalpick-up over the wave spectrum is not entirely satisfactory because ofthe variable signal gain over the spectrum; a low signal responseoccurring at the low frequency end of the wave spec'- trum and a highsignal response occurring at the high frequency end thereof.

It is an object of this invention, therefore, to provide an improvedloop antenna system for radio receiving apparatus;

It is another object of this invention, to provide a loop antenna systemfor radio receivers which substantially eliminates the undersirablecharacteristics common to loop antenna systems, now in commercial use.

It is a further object of this invention to provide a loop antennasystem which operates to effect maximum attenuation at some undesiredfrequency.

It is still another object of this invention to provide a loop antennasystem which operates to effect maximum attenuation at some undesiredfrequency and a substantially uniform signal response over a desiredwave band.

It is a particular object of this invention to provide a double loopantenna system which combines all of the advantages of a single loopantenna system for a radio receiver as to installation and compactness,with improved operating characteristics, to in all provide a compactradio receiver operating effectively to produce a substantially uniformsignal response over an entire band of frequencies.

Other objects, features and advantages of this invention will beapparent from the following description taken with the drawing in which:

Fig. l is a diagrammatic illustration of a representative radio receiversystem employing the double loop antenna system of the invention.

Fig. 2 is a circuit diagram illustrating a modified arrangement of theinvention.

Fig. 3 is a circuit diagram illustrating a further modification of theinvention.

Fig. 4 shows in curve form some of the operating characteristics of theantenna systems shown in Figs. 1, 2 and 3, and

Fig. 5 illustrates diagrammatically one method of assembling the loopsincluded in the antenna systems of Figs. 1, 2 and 3.

In practicing this invention there is provided a radio receiver systemincluding a pair of loop aerials, which are electrically connected in amanner to efiect complementary operating characteristics. In theoperation of the antenna system it is contemplated that one of the loopshave a higher inductance than the other with the circuit arrangement ofthe loops being such that the maximum signal pick-up for each loop willnot occur at the same wave frequencies, the loop having the lesserinductance being most efiective at the'high end of a band of frequenciesbeing received and the loop of larger inductance being most efiective atthe lower end of such band of frequencies. The loops are not limited asto their physical size and may vary in their relative structure.However, for the purpose of simplicity and convenience of explanationthe loops in the following description will be considered as havin thesame physical dimensions.

In the drawing similar characters of reference shall be used in thevarious figures to designate similar parts. With reference to Fig. 1,there is shown an antenna circuit with loops L1 and L2 and condenser C1in series connection. The voltages induced in the loops L1 and L2 by theincident wave are illustrated, for the purpose of clarity andconvenience, as being generated by generators E1 and E2, respectively.An inductance L3 and a capacity C3 are connected in series across theloop L2; the signals picked up being taken off across the capacity C3 byconductors l0 and II, conductor 10 connecting In in series with the grid[2 of the RF or converter tube 13, which constitutes the first tubestage in the receiver circuit. The remainder of the circuit issubstantially conventional including the RF or IF amplifier, thedetector, and the AF amplifier which are designated by the appropriateabbreviation in the illustration. A speaker I4 is connected to theaudio-frequency amplifier in a normal manner. The resistor I1 provides abias for the grid l2 and the capacitor 18 provides a low impedance pathto the ground.

Referring specifically to the antenna circuit of Fig. 1 the features ofthe invention are found in the novel arrangement and conditioning of theloops L1, L2 and capacity C1. The additional elements in this circuitmerely indicate a manner of connecting the loops L1 and L2 to autilization circuit, which is illustrated as including the elements L3and C3, the connecting terminals being shown at B-B. The antenna circuitper se is not tunable, but resonance of the system is obtained byvarying the tuning elements L3 or C3 in the utilization circuit. Thistuning may be obtained by a variation of the capacitance C3 with a fixedinductor L3, by a variation of the capacitance C3 with L; equal to zeroand L2 providing the necessary tuning inductance or by a variation ofthe inductor L3 with a fixed capacitance C3, the operation of theantenna system being equally effective regardless of which of the twoelements L3 or C3 is varied for resonant tuning. To better understandthe invention and its operation the various circuit relations will bediscussed in an analysis of the fundamental equations setting up theserelations.

.For convenience in the analysis, the antenna circuit shall refer toboth loop circuits, the utilization circuit to the circuit including L3and C3 and the antenna system shall include all circuits.

The circuit elements and factors associated with each loop shall besuitably designated to indicate such association.

Since the determination of the signal response of the antenna systemover a band of frequencies requires a determination of the character ofthe response, the analysis is predicated on the magnitude of theresponse and the conditions in the system affecting such magnitude. Itis readily seen that the voltage delivered across the loop L2 to theutilization circuit represents the magnitude of the signal response tobe reproduced by the receiving apparatus. This voltage, designated as E,can be expressed in terms of E1 and E2, which correspond to the voltagesinduced in L1 and L2, respectively. Since the voltages E1 and E2 areassumed as connected in series opposition,'the relation between them,neglecting dissipation, can be written as:

E1E2=(I1-I2) ('Z1+Z2) and the voltage E across loop L2 as:

E=E2+ (I1-I2) Z2 (2) By a proper substitution of I Equation 1 intoEquation 2 the expression for E becomes:

in which Equation M is the mutual coupling factor between loops L1 andL2; '7' is an operator equal to /1 and :21 1 representing the frequencyin cycles.

The signal response of the system is thus set up in terms of E1, E2 andthe impedances effected in the loop circuits. It is to be noted alsofrom the above equations for Z1 and Z2, that the loops L1 and L2 may berelatively positioned to obtain a mutual coupling factor of either plusor minus sign, Without afiecting the phase opposition of E1 and E2. Thischange in the sign of the coupling factor can be accomplished bychanging the clasps from an essentially coplanar position to anessentially coaxial osition or vice versa.

Equation 3 for E, therefore, can be written in a more applicable form bysubstituting therein equivalents for the voltages E1 and E2, which areinduced in the loops L1 and L2, respectively. Thus:

E =1=N fsin cos 9 and r i firs I E eN f s1n( cos 0 in which e is thestrength of the radio wave in volts per meter, S is the width of theloop in meters, f is the height of the loop in meters, N1 and N2 are thenumber of turns in L1 and L2, respectively, and 01, and 02 are theangles of the incident wave with respect 'to the planes of loops L1 andL2, respective-1y.

- Since it is contemplated in the invention touse loop dimensions which'are relatively small as compared to a wave length, the followingrelations can be written forEquations 6 and Twith only negligible error,in which where represents the frequency in cycles and C the velocity oflight.

The induced voltages E1 and E2 of Equations 6 and 7 then become (9) andThe function for designates a particular definite frequency and will behereinafter fully explained. In the present analysis, the loops L1 and112 are considered for simplicity as having similar physical dimensions,but it is to be understood that the invention is not restricted t'o'astructural similarity of the loops for its successful operation.

The expression efvi-Sfoi/C in Equations '9 and 10 represents a constantfor each loop L1 and. L2 so that these equations may be rewritten in amore simple form as and sible solutions for obtaining a zero value of E,and that the functions cos 02/ cos 01, N2/N1 and Z1/Zz are equallyimportant in determining the frequency at which E is equal to zero. Asnoted abov it is a particular feature of the invention to provide a loopantenna system which effects a maximum attenuation at some undesiredfrequency. Since maximum attenuation occurs when E is equal to zero,Equation 13 may be utilized to obtain an antenna system having certainoperating characteristics, by assuming therein particular values for thevariable functions. These characteristics may be graphically illustratedto indicate the operation of a particular system.

With reference to Fig. 4, curve III shows the operating characteristicsof a particular antenna system which was designed in accordance withthis invention. The values of E are represented as the ordinates and thefrequency ratio of Him, as the abscissae. Curve III represents asolution to a receiving situation in which it is desired to have maximumattenuation at some undesired intermediate frequency designated as f0,such as a frequency of 465 kc., which occurs below the the usualbroadcast range. The broadcast range or band is defined by a lowfrequency for and a high frequency I02, the voltage gain between theselimit being of a substantially constant value. Curve III, therefore,covers an antenna system which operates to effect a substantially fiatsignal gain over a broadcast range between the frequencies of for andI02, with maximum attenua tion of an undesired frequency in below thebroadcast range. curve III illustrates the operating characteristics ofonly one particular antenna system and that the invention is not to beso limited.

In the use of Equation 3 for determining the arrangement of the circuitelements to effect a particular desired result, such as shown by curveIII of Fig. 4, the loops L1 and L2, for simplicity, shall be consideredas having induced therein a maximum voltage E1 and E2, so that 91 and02, which represent the angles of the arriving wave with respect to theplanes of the loops L1 and L2, respectively, are each equal to zero.Since an antenna system efiecting a flat signal gain over a desired wavespectrum, with maximum attenuation at a frequency lying outside of suchspectrum is desired, a better understanding of the system can beobtained if Equation 3 is expressed in terms of the frequencies definingthe extent of the spectrum. Equation 4 is thus rewritten as where 1 2-.accmgm E,=TLN. (l6) for and E =T- -N2 (17 for As previously noted Thefactor for in Equations 16 and 17 thus can- It is to be understood that'r is equal to eJwsm/c.

in which In Equation '18 E becomes zero when the frequency, in is f0:for

NiK This value for in is obtained from Equation 18 by setting thenumerator portion equal to zero and solving for ,1. Since I is thegeneral designation for the incident wave, the value of J at which Ebecomes zero will be the frequency of maximum attenuation and hence in.By plotting Equation 18 as a function of f/fm there is obtained curveIII of Fig. 4, the desired flat signal gain occurring over the frequencyrange 1m to I02 and maximum attenuation f an undesired frequencyoccurring outside of this range. It is thus readily apparent that oncethe equational relations of the system are known, the respectiveoccurrences of f0 and for are rapidly ascertained.

For a better understanding of Equations 18 and 19, which express thecombined effect of the complementary operation of L1 and L2 in providingfor a system having the operating characteristics shown in curve III inFig. 4, there is also shown in Fig. 4 curve, II, which indicates thesignal response of L2 alone; when L1 is disconnected therefrom. It is tobe noted from this curve that L2 operating alone effects the usualoperating characteristics of a single loop antenna, namely, a signalgain in direct proportion to the frequency. A comparison of curve IIwith curve III illustrates the operating advantages obtained by theadditional loop L1 in circuit with L2. It is tobe understood, however,that curve II does not represent the voltage contributed by E2 to E whenthe loops L1 and L2 are connected together in a complementary operatingrelation.

Thus with reference to the system of Fig. 1 the voltage contributed to Eby E: acting alone in the system, is found to be 221 z,+z2 20 whichexpression plotted as a function of Him, is shown in curve IV.Conversely the contribution to E from E1 acting alone in the system ofFig. 1 becomes E zg Z1+Z (21) the voltage Em plotted as a function off/for being illustrated in curve I.. Since the sum of the voltagecontributions Em and EZA is equal to the signal voltage E, it is obviousthat the sum of curves I and IV will yieldcurve III. It is to be notedalso that the'frequency f ofmaximum attenuation occurs when E1A=E21nWith further reference to Equation 19, itis seen that T01 occurs at afrequency higher than fa, when K, the coupling factor is positive.However, when K is negative the occurrence of in is dependent upon theabsolute value of K, and i111 therefore may fall either above or belowI01. For a zero value of K, f0 will occur at a frequency lower than for.

However, upon a reversal of one only of the generators E1 or E2 so thatthe generators are in aiding connection, the equations for curve IIIwill not provide for a substantiallyuniform or flat signal gain over thedesired frequency range fin-T02 (Fig. .4). occur at a frequency higherthan in and phase or'adjustment changes of K will not effect anoccurrence of f0 below I01, as will now be described.

In the system of Fig. 1, therefore, let it be assumed that the polarityof one of the generators E1 or E2 is reversed. The equationscorresponding to the Equations of 18 and 19 are then found to be and Ebecomes zero when f0: fol

[ li-KN Nd: KN

which value of f0 occurs within .the zone of substantially uniformsignal gain, as shown .in curve III, Fig. 4, and hence is of a highervalue than fol. With the generators E1 and E2 in aiding connection thesystem is thus seen to be applicable to a condition where asubstantially uniform signal gain over the entire frequency range is notessential, and wh re a troublesome signal occurs at a frequency higherthan I01.

In Fig. 2 there is shown a modified arrangement of the invention, inwhich L1, 01 and C2 are connected in series to form a primary circuitand L2 and C2 are connected in series to make up a secondary circuit. Itis thus seen that the common coupling paths between the primary andsecondary circuits include C2 and the inductive coupling between L1 andL2. The utilization circuit which includes L3 and C3, is in seriesconnection with the secondary circuit; the signal pick-up being takenofi across C2 to the receiving apparatus as above described inconnection with Fig. 1. The coupling capacitor C2 in the system Underthis condition is Will of Fig. 2, which is not included in the system ofFig. 1, provides for greater flexibility in the selection of theoccurrence of the frequency of maximum attenuation, since C2 inconjunction with C1 allows for a greater variation in the couplingbetween L1 and L2. As noted above I0 is moved with respect to I01 withvariation in the coupling K between loops L1 and L2 so that relativemovement between .f0 and fo1 is eifected by variation only of,thecoupling factor. Since C2 provides for additional coupling betweenthe loops L1 and L2, it is apparent that a corresponding greatervariation is obtained in the respective occurrences of I0 and i111.

Since the operation of the system of Fig. 2 is substantially similar tothat of Fig. 1, only the pertinent equations will be referred to in thefollowing circuit analysis. 'Thus with the generators E1 and E2 poled inopposition, the iunda- 1 M between the two loops.

mental equations for the induced voltages E1 and E2 in the system ofFig. 2 are From Equations .24 and 25 the current I2- in the loop L2 isfound to be By assuming, for simplicity, that the elements in the systemare dissipationless the zero value for the current I2 may be written asI2=0=E1ZmE2Z1 By suitable substitution Equation 28 is reduced toequivalent .factors in a manner similar to that followed in connectionwith Equation 3 to provide for a determination of ft, the frequency ofmaximum attenuation. By this procedure f0 is found The sign to beapplied to A in Equation 29 is the sign of the mutual inductance Mbetween the loops L1 and L2. A curve predicated on Equation 27 andindicating the response characteristics across the terminals B-B of thesystem in Fig. 2, is substantially similar to the curve III of Fig. 4.As noted above, however, the circuit element C2 in the system of Fig. 2permits greater latitude in the occurrence of f0 with respect to foi, ascompared to the system of Fig. 1, which does not include this element.In the system of Fig. 2, as shown by Equation 29, To can be made tooccur either above or below In, regardless of the polarity of the mutualcoupling between L1 and L2.

Thus in the system of Fig. 2 let it be assumed that one of thegenerators E1 or E2 is reversed. The equation corresponding to Equation29 be- It is seen, therefore, that the system represented by Equation 30eifects an occurrence of in at a frequency higher than I01.

With reference to Fig. 3 there is shown a further modification of theinvention. In this system the loops L1 and Lzare not in directelectrical connection, the electrical energy from L1 being transferredto L2 by the common inductance The primary loop circuit includes L1 and01 connected in series, and the secondary loop includes only L2. Thesignal response is taken off across L2, to the utilization circuit,which is suitably connected with a radio receiving apparatus in a mannersimilar'to that hereinabove described in connection with the systems ofFigs. 1 and 2. The operation of the system otFig. 3 is essentially thesame as -the operation of the systems of Figs. :1 andz.

The system of Fig. 3' may be operated to effect a uniform or flat signalresponse over a given frequency range which i substantially similar tothat indicated in curve III of Fig. 4. However this flat gain can onlybe obtained when the mutual M between L1 and L2 is of plus sign and theinduced voltages E1 and E2 are in phase opposition. The fundamentalequations relating to this system are The relations expressed inEquation 33 can only be satisfied when the loops L1 and L2 are arrangedsubstantially in coplanar relation and these relations do not hold whenthe loops approach a relatively coaxial position. This distinction isconsistent with the conditions above noted and specified in Equations 31and 32, namely a positive mutual M and opposing voltages E1 and E2. Whenthese conditions in Equations 31 and 32 are not satisfied Jo will occurat a frequency higher than fol. It is thus seen that the coplanarrelation indicated in Equations 31 and 32 will provide for an occurrenceof f at a frequency which may be either above or below 101, dependingupon the polarity of the induced voltages and the sign of the mutual Mbetween the loops.

In all of the above equations relative to the antenna systemsillustrated in Figs. 1, 2 and 3, it has been assumed that the angles 01and 02, between the plane of each loop L1 and L2, respectively, and theincident wave, were identical and equal to zero. It is to be understood,therefore, that any changes in the angular positioning of the loops L1and L2 with respect to the incident wave and with respect to each other,will effect important changes in the occurrence of ft with respect to fmas is most clearly exemplified in Equations 13 and I4. I 'hese changesmay be readily effected in the systems of Figs. 1, 2 and 3 by arrangingthe loops L1 and L2, thereof, as shown in Fig. 5.

With reference to Fig. 5, L2 is fixedly mounted to a suitable portion ofthe radio frame I9 with L1 hinged as at 2| to the loop L2 so as to berotatable about L2 in an arc of at least 180. It is readily apparentthat rotation of L1 with respect to L2 will operate to eliminate certainstations which lie either above or below 2'01.

The invention thus provides for a compact selfcontained loop antennasystem which provides maximum attenuation at intermediate frequenciesand a relative uniform response to signals over a range of frequenciesto be received. It is to be noted also that substantially completeattenuation may be provided at a particular frequency at which the radioreceiver is sensitive and substantially less attenuation at otherfrequencies to which the receiver is responsive. Resonance of the systemmay be obtained by a variation of the capacitance G3 with the inductorLa fixed, by a variation of the capacitance C3 with La out of thecircuit and L2 suitably increased to provide the necessary inductance,or by a variation of the inductor L with the capacitance C3 fixed,Without affecting the efficient operation of the system. The antennasystem of this invention is thus seen to be very flexible in itsapplication and adapted to effect operating charactistics incorrespondence with a particular troublesome receiving difiiculty so asto counteract or eliminate such difiiculty.

It is to be understood that only specific embodiments of the inventionhave been illustrated and described herein and that modifications andalterations in the arrangement of the invention may be made withoutdeparting from its full intended scope, as defined by the claims whichare appended hereto.

I claim:

1. A radio antenna system including a pair of loops having differentinductive characteristics, with said loops connected in series, and acondenser of fixed capacity connected in series with said loops, saidloops and condenser being connected to effect complementary operatingcharacteristics, the combined effect of such operating characteristicsproviding for maximum attenuation at a particular undesired frequencyand for a substantially flat signal response over a desired group offrequencies, and a utilization circuit having an inductor and acapacitor connected across the loop of lower inductance and adapted toeffect resonance of said antenna system by varying at least one of theelements in said circuit.

2. A radio antenna system including a pair of loops of differentinductive characteristics connected in series, and a condenser in serieswith the loop of larger inductance, said loops and condenser and theconnections therebetween providing for a signal response across the loopof lower inductance which is relatively uniform over a larger group offrequencies and attenuated over a restricted group of frequencies, and asecond condenser connected between said loops and in series with saidfirst condenser, said second condenser being adapted to vary thecoupling between said loops to effect a shifting of said restrictedgroup of frequencies relative to said larger group of frequencies.

3. A radio antenna system including a pair of loops of differentinductive characteristics connected in series, and a condenser in serieswith the loop of larger inductance, a second condenser connected betweensaid loops and in series with said first condenser, and a utilizationcircuit having an inductor and a capacitor connected across the loop oflower inductance for tuning said antenna system by varying at least oneof the elements in said utilization circuit.

DAVID E. SPARKS.

